The invention is based on a fuel injection pump for internal combustion engines. Such fuel injection pumps are embodied either as so-called in-line injection pumps, in which there is a separate pump element for each cylinder of the engine and these pump elements are disposed in a line, or as so-called reciprocating slide pumps, which are primarily intended for heavy-duty use, for instance in trucks.
In the second type of fuel injection pumps, not only exact metering of the injection quantity but a very accurate setting of the instant of injection onset are attained by axially displacing a control slide provided on each pump piston, and/or by rotating the pump piston.
In both types of injection pumps, an overflow of the highpressure fuel at the metering control edges causes heating of this returning fuel, which also heats the fresh fuel located in the suction chamber. The heating changes the physical properties such as density and compressibility of the fuel, so that the quantity of fuel metered per pump stroke and its energy content both vary as well. In the ensuing injection, temperature differences in the fuel supplied therefore leads to changes in the output of the engine cylinders.
Near the entrance of the fuel inlet conduit, the fuel temperature in the suction chamber is still relatively low, because of the high proportion of fresh fuel, but with increasing distance from the entry, this temperature rises until it has attained a maximum in the region of the fuel outflow from the suction chamber. The fuel temperatures in the various pump work chambers of the injection pump are correspondingly variable, with the abovediscussed consequences.
To avoid nonuniform cylinder outputs, such fuel injection pumps have separate suction chambers, from which the injection pump is supplied with fuel. By means of equal volumetric flows of fuel in all the separate suction chambers, the fuel temperature can be kept the same in all the separate suction chambers.
In a known fuel injection pump of the generic type involved of a reciprocating slide pump (German Offenlegungsschrift 35 46 222), the volumetric fuel flows in the separate suction chambers are regulated by providing radial branch bores, acting as throttles, in the wall of a tube that acts as a fuel inlet conduit and tapers in steps in the direction of the flow; there is one bore in each step, and via an associated connecting conduit, it communicates with an associated separate suction chamber. The cross sections of these radial branch bores and their length are adapted to one another such that the volumetric flow through the bores is the same for the all the pump elements. The rotational position of the tube is defined by a fixation screw running in the housing; this position must meet very high demands for accuracy, so that particularly with reciprocating slide pumps, it must first be assured that the connecting conduit between the throttle opening in the tube and separate suction chamber and the throttle opening itself are precisely in alignment, because otherwise the volumetric flow is reduced, and second that there be sufficient sealing between the tube and the pump housing to prevent leakage, which would also affect the volumetric flow. For the same reason, the tube must be fitted very accurately into the pump housing. Especially in pumps having a high number of cylinders, this means high production expense and hence high production costs.
Production expenses and costs are also increased because of the fact that metering tubes for in-line pumps must be embodied differently from those for reciprocating slide pumps, so that in mass production, two different metering tubes must be produced.
Besides an accurate positioning of the tube in the pump housing and fastening the tube, there are other problems. Since the position of the tube must meet high demands for accuracy, the fastening must be correspondingly reliably embodied to prevent shifting and twisting. Because the fuel flow is deflected sharply as it enters the suction chamber, there is a danger of cavitation damage to the surrounding material.
In known injection systems of this generic type, the connecting conduit between the metering tube and an electric shutoff means must be disposed at a particular site. This has a disadvantage of determining the positioning of the electric shutoff means on the pump housing, so that it cannot be freely selected.